This invention relates to an image formation system using an electrophotographic technology, such as a copier, facsimile, or a printer, and more particularly to an image formation system using an intermediate transfer technique involving primary and secondary transfer steps with an intermediate transfer body, etc., intervening and an intermediate transfer body used with the image formation system.
A method of once primarily transferring a toner image (developed image) formed on an image support such as a photosensitive drum onto an intermediate transfer body other than transfer paper and then secondarily transferring the image onto transfer paper for providing a copy image is known as an image formation method (transfer method) in a color image formation system such as an electrophotographic copier.
It is known that this method can be used to suppress occurrence of a multiple transfer failure and a color registration shift caused by many factors of a transfer paper hold state, transfer paper thickness or firmness, a transfer paper surface property, etc.
FIG. 10 is a schematic representation of an image formation system in a related art using an intermediate transfer body incorporating the Invention.
In the figure, an image formation system F has a UI (user interface) and a transparent platen glass 2 on which an original (not shown) is placed.
The original placed on the platen glass 2 is illuminated by a light source 4 of an original light unit 3. Original-reflected light is reflected on a first mirror 5 of the original light unit 3 and a second mirror 7 and a third mirror 8 of a mirror unit 6, passes through an image formation lens 9, and is read as analog signals of R, G, and B by a CCD (charge-coupled device).
The image signals read by the CCD are input to an IPS controlled by a controller C. Image read data output means 11 of the IPS converts the input read image signals from analog form into digital form. Image data output means 12, which has an image memory 13, converts the digital data provided by the image read data output means 11 into Y, M, C, X image data, performs data processing of density correction, scaling correction, etc., and outputs the resultant data as write image data (laser drive data).
A laser drive signal output unit 14 outputs a laser drive signal of an image responsive to the image data output from the IPS to an ROS (optical scanner, namely, a latent image formation unit) at a predetermined timing. The ROS outputs a laser beam L modulated by the laser drive signal.
An image support 16 which rotates in charged by a charger 17, then an electrostatic latent image is written onto the image support 16 at a latent image write position Q1 by the laser bean L and the electrostatic latent image is developed to a toner image by a rotary developing unit 18 having K (black), Y (yellow), M (magenta), and C (cyan) color developing devices 18k, 18y, 18m, and 18c rotating together with a rotation shaft 18a in a developing area Q2. The developed toner image is primarily transferred to an intermediate transfer belt B by means of a primary transfer roll (primary transfer device) in a primary transfer area Q3.
Residual toner on the surface of the image support 16 passing through the primary transfer area Q3 is removed by an image support cleaner 20.
The intermediate transfer belt B is supported by belt support rolls 26-29 of a drive roll 26, a tension roll 27, an idler roll 28, and an inner secondary transfer roll (backup roll) 29 for rotation.
A secondary transfer device T2 is made up of the inner secondary transfer roll 29, an outer secondary transfer roll 30 placed between a distant position from the inner secondary transfer roll 29 and a near position at which the outer secondary transfer roll 30 is pressed against the inner secondary transfer roll 29 so that the outer secondary transfer roll 30 can be moved away from and toward the inner secondary transfer roll 29, and an electrode roll 31 coming in contact with the inner secondary transfer roll 29. The secondary transfer device T2 is driven by a power supply circuit 32 controlled by the controller C.
The secondary transfer device T2 secondarily transfers the toner images primarily transferred onto the intermediate transfer belt B in overlapped relation to a transfer material S passing through a secondary transfer area Q4 formed in a nip (contact area) between the outer secondary transfer roll 30 and the intermediate transfer belt B.
Toner deposited on the surface of the outer secondary transfer roll 30 is removed by a secondary transfer roll cleaner 33. A belt cleaner 34 in placed downstream from the secondary transfer area Q4 along the surface of the intermediate transfer belt B.
A belt position sensor SN1 is provided for detecting a position detection mark (not shown) to detect a rotation position of the intermediate transfer belt B. The latent image write timing onto the image support 16 is controlled by a position detection signal of the intermediate transfer belt B with very high accuracy output from the belt position sensor SN1.
The transfer material S stored in a paper feed tray 41 is taken out by a pickup roll 42, is separated one sheet at a time by a handling roller 43, is once stopped at a registration roll 44, and is transported from a guide transport passage 45 to the secondary transfer area Q4 at a predetermined timing. The transfer material S to which the toner image is secondarily transferred when the transfer material S passed through the secondary transfer area Q4 is transported through a sheet guide 46 and on a sheet transfer belt 47 to a fixing area Q6. When the transfer material S passes through the fixing area Q6, the secondarily transferred toner image onto the transfer material S is fixed by a pair of fixing roll a of a fuser 48 and the transfer material S is diacharged to a paper discharge tray 49.
The components 41 to 47 make up a transfer material transporter (42-47).
The image formation system incorporating the invention shown in FIG. 10 uses the intermediate transfer belt B as an intermediate transfer body, but the invention can also be applied to an image formation system using an intermediate transfer drum in place of the intermediate transfer belt B.
In the image formation system using an intermediate transfer body as described above, hitherto a substance comprising a filler such as carbon or a metal compound dispersed in a polymeric material as a conductive agent has been used an the intermediate transfer body. (The Unexamined Japanese Patent Application Publication No. Hei 8-320622).
It is known that there is a close relation between the volume resistivity of the intermediate tranafer body and the toner image quality.
(When Volume Resistivity of Intermediate Transfer Body, .rho., is low)
When the volume resistivity of the intermediate transfer body, .rho., is too low (.rho..ltoreq.10.sup.6 .OMEGA.cm), toner strikingly scatters at the transfer time and the image quality is degraded (The Unexamined Japanese Patent Application Publication No. Hei 8-248779). The possible reason is that when the volume resistivity of the intermediate transfer body is too low, a transfer electric field is easily applied to an area with no toner layer by the action of transfer current and transfer electric field produced by a primary transfer roll and thus a transfer area spreads and toner scatters and is transferred because of the effect.
(When Volume Resistivity of Intermediate Transfer Body is Intermediate)
The current intermediate transfer body of the image formation system in the actual use has an intermediate value of volume resistivity (10.sup.6 .OMEGA.cm.ltoreq..rho..ltoreq.10.sup.14 .OMEGA.cm).
Such an image formation system has charges attenuated properly because of the semiconductivity of the intermediate transfer body. That is, an average value of the volume resistivity of the intermediate transfer body lies in the range in which the charges are attenuated properly (the volume resistivity is in a proper range), thus consecutive image formation can be executed without using an electricity removal member.
However, if the average value of the volume resistivity of the intermediate transfer body lies in the proper range (the range in which the charges are attenuated properly), the intermediate transfer body in related art causes the following problems:
If a filler such as carbon or a metal compound is dispersed in a polymeric resin, resistance variations in the intermediate transfer body caused by the dispersion state of the filler are an order of magnitude larger, low resistance of the intermediate transfer body caused by electric breakdown of a minute polymeric resin part between fillers, filler re-arrangement caused by energization, and the like occur with time. Thus, when printout is produced, the volume resistivity of the intermediate transfer body is placed out of a good volume resistivity range partially or on a whole with time, degrading the image quality.
(When Volume Resistivity of Intermediate Transfer Body, .rho., is High)
When the volume resistivity of the intermediate transfer body, .rho., is high (.rho.&gt;10.sup.14 .OMEGA.cm), the charge retainability of the intermediate transfer body in a toner image area is enhanced and an electric field required for transfer can be applied to toner appropriately. On the other hand, charge move on the interediate transfer body surface of an adjacent non-image part and internal charge move decrease, thus toner transfer to the area in the primary transfer and secondary transfer becomes hard to occur.
Thus, when the volume resistivity of the intermediate transfer body is high, toner less scatters and a good-quality toner formation image can be provided. In this case, however, a step of removing charges accumulated on the intermediate transfer body becomes necessary after toner transfer, and it is difficult to uniformly remove the charges accumulated on the intermediate transfer body at the removal step, thus the art is not in the actual use. The reason why the art is not in the actual use is as follows:
To provide the charge (electricity) removal step, it is easily assumed that a corotron, an electricity removal roll, etc., is used as an electricity removal member.
To use an electricity removal roll as the electricity removal member to remove the charges (electricity) accumulated on the intermediate transfer body, an electricity removal roll with AC bias voltage applied may be used. In this case, since there are asperities, etc., on the intermediate transfer body surface and the electricity removal roll surface, the full surface of the intermediate transfer body which rotates cannot be brought into contact with the electricity removal roll grounded. Electricity on the intermediate transfer body portion not coming in contact with the electricity removal roll is hard to remove in a state in which the volume resistivity of the intermediate transfer body is high. That is, when the volume resistivity of the intermediate transfer body is high, it is difficult to reliably remove electricity on the full surface of the intermediate transfer body in a short time for which the intermediate transfer body passes through the electricity removal member. If uneven electricity removal occurs on the intermediate transfer body surface, inconsistencies in density responsive to the uneven electricity removal occur in a toner image primarily transferred to the intermediate transfer body surface at the next image formation time. Because of the electricity removal step of discharging, an expensive high-voltage power supply for electricity removal also becomes necessary.
To use a corotron to remove electricity, a large amount of ozone occurs, image quality defect based on uneven electricity removal caused by deposit on the corotron occurs, and a discharge product of NOx, O3, or any other ozone reaction product is deposited on the intermediate transfer body.
To use an intermediate transfer body of high resistivity with the volume resistivity .rho. exceeding 10.sup.14 .OMEGA.cm as described above, an electricity removal member for removing electricity after the termination of secondary transfer becomes necessary. Particularly, the higher the value of .rho., the more difficult to uniformly remove electricity on the full surface of the intermediate transfer body if the electricity removal member is used.
Therefore, the intermediate transfer body having high volume resistivity is useful considering only the transfer effect, but requires an electricity remove step and it is hard to uniformly remove electricity on the intermediate transfer body. Thus, hitherto an image formation system using an intermediate transfer body having high resistivity (volume resistivity .rho. is greater than 10.sup.14 .OMEGA.cm) has not yet been in the actual use.
Arts (J01) to (J03) described later are known as those using an intermediate transfer body with volume resistivity lowering if physical stimulation is given in conventional image formation systems. The reason why an intermediate transfer body whose resistance value lowers is used in (J01) to (J03) is that electricity on the intermediate transfer body to which primary transfer is executed by a special method is removed before secondary transfer in (J01) and that lowering the resistance value of the intermediate transfer body is used to execute secondary transfer in (J02) and (J03).
That is, (J01) to (J03) provide image formation systems each using an intermediate transfer body whose resistance value lowers to execute special transfer and are different arts from those of image formation systems for executing transfer by normal transfer electric field. That is, (J01) to (J03) are idea arts and are not in the actual use. Moreover, the following problems are involved:
(J01) Art described in the Unexamined Japanese Patent Application Publication No. Hei 7-146616 (see FIG. 11)
FIGS. 11A to 11F are schematic representations of an art described in the Unexamined Japanese Patent Application Publication No. Hei 7-146616.
In the figure, Q1 denotes a primary transfer area, Q2 denotes a secondary transfer area, and 01a and 01b (FIG. 11B) denote a conductive rubber layer 01a with carbon mixed and a silicone rubber layer 01b with a charge generation agent mixed as surface layers of a blanket 01 (FIG. 11A).
In FIG. 11C, the blanket 01 (intermediate transfer body) having photoconductivity is used as an intermediate transfer body and the surface of the blanket 01 is charged to polarity (+) opposite to charge polarity (-) of toner. In FIG. 11D, toner is primarily transferred from a photosensitive drum 02 to the intermediate transfer body blanket 01 by a static electricity force caused by the charge. In FIG. 11E, after the primary transfer and before secondary transfer, light L is applied to the intermediate transfer blanket 01 and the primarily transferred toner image on the surface of the blanket for removing electricity (see FIG. 12A). In FIG. 11F, the bonding force between the surface of the blanket 01 and the toner is weakened by removing electricity, thus secondary transfer is easily executed by application of light.
However, in the art, as shown in FIG. 12B, the surface of the blanket 01 is charged to the polarity opposite to the toner charge polarity on the photosensitive body 02 as a stage before primary transfer is executed from the photosensitive body 02 to the intermediate transfer blanket 01, thus toner transfer occurs in the area before the primary transfer area; degradation of image quality caused by image disorder, bleeding, etc., cannot be prevented.
FIGS. 12A to 12D are schematic representations of a problem arising when the art described in the Unexamined Japanese Patent Application Publication No. Hei 7-146616 (art (J01)) is applied to a multiple transfer system of color image in different color toners.
In FIG. 12, the first color toner is primarily transferred to the intermediate transfer blanket 01, then the first color toner on the blanket 01 is charged to the polarity opposite to the toner charge polarity on the photosensitive body 02 for primary transfer of the second color toner (FIG. 12C) and the toner on the photosensitive body 02 is transferred to the blanket 01. At this time, a sufficient transfer electric fluid to transfer the toner on the photosensitive body 02 is applied between the photosensitive body 02 and the blanket 01, so that the second color toner on the photosensitive body 02 is transferred to the blanket 01. Conversely, the first color toner of the opposite polarity on the blanket 01 is transferred to the photosensitive body 02 (FIG. 12D). Therefore, if the art described in (J01) is applied to the multiple transfer system of color image, it is very difficult to provide a good-quality color image by multiple transfer.
(J02) Art described in the Unexamined Japanese Patent Application Publication No. Hei 5-257398
FIGS. 13A to 13D are schematic representations of an art described in the Unexamined Japanese Patent Application Publication No. Hei 5-257398.
The Unexamined Japanese Patent Application Publication No. Hei 5-257398 proposes a method intended for preventing transfer in a before-transfer area in FIGS. 13A and 13B, the method of applying precharges of the same polarity as a toner charge polarity onto the surface of a photoconductive intermediate transfer body 01 by a before-transfer charger 03 before primary transfer is executed as shown in FIG. 13C and applying light from the rear face of the intermediate transfer body 01 in a primary transfer area Q1 for executing primary transfer as shown in FIG. 13D.
This method can reduce transfer in the before-transfer area, but can provide good transfer images of only single-color images and is hard to provide a clear full color image.
FIGS. 14A and 14B are schematic representations of a problem arising when a developed image of the second color is transferred by the art described in the Japanese Patent Application Publication No. Hei 5-257398.
In FIG. 14, precharges are given also including the toner image of the first color already transferred onto an intermediate transfer body 01 (FIG. 14A) and light is applied from the rear face in a primary transfer area Q1 for executing transfer (FIG. 14B). At this time, charges on the intermediate transfer body 01 are removed, but the toner charges of the first color are not removed. Resultantly, an electric field produced by the unremoved toner charges of the first color causes the second color toner to scatter and unevenness to occur.
(J03) Art described in the Unexamined Japanese Patent Application Publication No. Hei 6-282181
FIGS. 15A and 15B are schematic representations of an art using an intermediate transfer body whose resistance value lowers by light application and applying light for executing transfer, described In the Unexamined Japanese Patent Application Publication No. Hei 6-282101. FIGS. 16A and 162 are schematic representations of an art using an intermediate transfer body whose resistance value lowers by applying pressure and applying pressure for executing transfer, described in the Unexamined Japanese Patent Application Publication No. Hei 6-282181.
In FIG. 15, a transfer material 04 such as paper is electrostatically held on an intermediate transfer body 01, light is applied from the rear face of the intermediate transfer body 01 in a transfer area Q1, and a transfer electric field produced by holes generated in a charge generation layer 01c is applied for transferring a toner image on a photosensitive drum 02. In this method, the resistance of the intermediate transfer body 01 is lowered by the action of light, etc, in the transfer area Q1 and a transfer electric field is applied, thus resistance becomes low in the axial direction of the photosensitive drum 02 (in a direction perpendicular to the move direction of paper). Thus, it becomes hard to prevent scatter and bleeding of a toner image in the axial direction.
FIG. 16 shows an art of lowering resistance by applying pressure rather than light in FIG. 15 and executing transfer. The art shown in FIG. 16 also involves a similar problem to that of the art shown in FIG. 15. In addition, it is hard to applying uniform pressure to an intermediate transfer body because of bend of a shaft of an intermediate transfer body support roll, asperities on the surface, etc., and transfer unevenness is caused by pressure distribution unevenness.